Method of coloring walnut sapwood using hydroquinone



Patented Aug. 1, 1950 METHOD OF COLORING WALNUT SAPWOOD USING HYDROQUIN ONE Eduard Farber, Washington, 13.9., assignor, by mesne assignments, to AmericanWa u Manu facturers As o iati n, nc" Chic o, 111,, a co poration of Illinois N Drawing. Application November 19, 1947, Serial No. 787,040

1 Claim. (Cl. 8-6.5)

Thepresent invention relates to a method of and to make possible an expansion of the uti i a tion of the same, it wouldbe highly desirable to convert the light color or the sapwood into a color resembling that of the darker heartwood.

It is well known that a superficial stain ng of this walnut sapwood can be effected to pr duce a more desirable, darker shade. However, such superficial staining can be accomplished only on finished articles made of waln apw od, and is not applicable to unfinished lum r. S hs a m ing applied to unfinished walnut sapwood lumber could not withstand subsequent finishing operations, such as planing and sanding. Such finishing operations would entirely remove any superficial staining which penetrates only to a negligible depth. Moreover, superficial staining applied to finished articles ultimately would be removed at least in part by chipping or splintering and may be removed by ordinary wear, even when protected by application of protective coat lugs and the like. Regardless of this insumcient The art has long recognized the need of an improved method of coloring walnut sapwood in the desirable shade resembling that of the heart- Wood. However, no means of treating the sapwood of the tree before or after rufiing into lumber or veneer exists which effects a coloration penetrating to depths suirlcient to prevent removal of the coloring upon subsequent working, for example, shaping, planing and sanding. Any satisfactory treatment resulting in a permanent coloring of the sapwood would require penetration of the coloring to a depth at leastsuflicient to prevent removal during finishing operations, and in some instances would require penetration throughout the entire thickness of the pieces treated, for example, the coloring of sapwood in veneer hitches.

It is therefore, a primary object and purpose of the present invention to provide a novel method for coloring walnut sapwood whereby it resembles in'shade the color of walnut heartwood. A further object of the invention is to provide a method of coloring walnut sapwood in which the coloring is permanently effected in suificient depth of penetration.

A further object or the invention is to provide a method of coloring walnut sapwood to resemble in shade the color of walnut heartwood in which the coloration is .eifected to any desired depth, n lud n c mp ete coloration through th en i thickness of the sapwood portion of the naterial treated. i

A still fu ther object of the present invention is to provide a method of coloring walnut sap- W d to s h a epth th t su equ nt operation exces ve W r, or e en chippin or spl terin will not destroy the color by removal thereof.

The method oi the present invention is applicable particularly to walnut sapwood in the form or unfinished ll mber or any other bull; form, including solid hitches or veneer flitches formed by slicing the solid ditches. However, the method is also applicable to any and all finished pieces of lumber and may also be applied to ished articles constructed from such lumber.

The method of the present invention in general comprises subjecting walnut sapwood to the action of certain agents which penetrate the wood readily and which react in some manner with the wood and/or each other to produce the desired coloration of the sapwood. Essentially, the method comprises treating the walnut sapwood, first with a solution of a substance which is Water soluble at ordinary and/or somewhat elevated temperatures, not precipitated by Wood extractives, and which is reactive to atmospheric oxyge particularly under alkaline conditions The thus treated wood is thereafter subleoted to the action of a substance in the gaseous or liquid phase which will produce an alkaline reaction in the wood. Such substance is characterized by high vapor tension and high solubility in water in which it produces a definitely alkaline reajce tion. If theyolatile alkaline-reacting substance used is the liquid phase or in solution, the wood to be treatedis preferably placed above the surface of the liquid thereby exposing it to an atmosphere containing the vapors of the reagent. In other words, a substance existing ,as a gas at ordinary temperature and pressure may be en}- ployed, or a solution of such a gas or a volatile] iquid having sufilcient vapor pressureat ordin ry te pe a ure and pr u e to c e an a nosphere containing the vapors off the reagent a o th iq be us th se nd step of the method, the wood is preferably contacted by the vapors of the alkaline-reacting substance rather than adopting the alternative of immers ing the wood in a solution of the same in order to avoid the possible extraction of the first reagent which has previously been absorbed into the wood. However, satisfactory results are obtained by spraying the alkaline-reacting substance in liquid form onto the wood or by immersing the wood in the liquid containing the alkaline-reacting substance.

Specifically, the first step in the process of the present invention involves subjecting the sapwood to the action of a polyhydroxy-benzene selected from the group consisting of hydroquinone and pyrogallol and mixtures thereof in any proportions. Preferably, the wood is treated with such polyhydroxy-benzenes in the form of aqueous solutions. The use of such aqueous solutions is preferred, since they have been found to be most satisfactory, and are by far the most economical. When aqueous solutions of these polyhydroxy-benzenes are utilized, any

desired concentration of these substances up to i satisfactory results are obtained therewith and higher concentrations present no advantage and would be somewhat wasteful. For example, concentrations from a few tenths of 1% to 3% are eminently suitable. The temperature at which the sapwood is subjected to the action of k the hydroquinone or pyrogallol can be widely varied and does not affect to any extent the result of this first step in the method of the present invention. The temperature selected will, of course, operate as a factor in determining the time required for the treatment in order to accomplish absorption of the polyhydroxybenzene solution into the wood to any given depth. When aqueous solutions of the polyhY- droxy-benzenes are utilized, temperatures ranging from ordinary atmospheric up to about 100 C. are preferred, although any temperature at which the solution remains in liquid phase may be employed. So also the concentration of the polyhydroxy-benzene solution will affect or control to some extent the time required for the penetration of the solution to a given depth.

The sapwood may be subjected to treatment with the solutions of hydroquinone and pyrogallol or mixtures in almost any desired manner, including immersion, steepage, dipping of the wood in the solutions, and spraying and/or brushing of the solutions on the sapwood material. When the treatment is carried out by steeping or dipping, the usual means for producing movement of the liquid around the wood may be provided, that is, the solution may be stirred or circulated by pumping means or the like.

Treatment of the walnut sapwood at ordinary atmospheric pressure with a solution of hydroquinone or pyrogallol will produce the necessary or required depth of penetration of the wood regardless of thickness as long as the time of treatment is prolonged for greater thicknesses or depths of penetration. On the other hand, the use of vacuum followed by pressure makes it possible to carry out the impregnation in about one hour or less for even the thickest pieces of sapwood material which would normally be treated. In general, at ordinary pressure the wood will be sufficiently penetrated by the polyhydroxy-benzene solution within a period of a. few hours, particularly where such solutions are heated.

It has been found necessary to maintain the pH of the polyhydroXy-benzene solutions below a pH of 7, or in other words on the acid side during the first step of the method. This adjustment of the pH of the solution may be effected by the addition of buffering agents such as alkali metal acid sulfites, acid phosphates, and the like, or b the addition to the solution of small amounts of strong acids, such as sulfuric acid.

After the walnut sapwood has been thus treated with the solution of polyhydroxy-benzenes, it is subjected to the action of the volatile alkaline-reacting substance, preferably by exposure to an atmosphere containing vapors of the same. Ammonia in dilute solutions, in concentrated solutions, in the gaseous phase, has been found to be the preferred volatile alkalinereacting substance for effecting the second step of treatment in the process of the invention. The ammonia in vapor form is readily absorbed by the sapwood and dissolves in the previously absorbed polyhydroxy-benzene solution, penetrating throughout thethickness thereof under ordinary atmospheric pressure, especially when the wood directly out of a heated solution of polyhydroxy-benzene is exposed to the ammonia. If desired, the atmosphere containing the ammonia vapors may be heated and/or humidified to aid in penetration'of the wood with the ammonia and where necessary to avoid cooling or premature drying of the wood. On the other hand, drying of the wool may be effected simultaneously with the ammonia treatment by using gaseous ammonia in. an ordinary wood drying kiln, to which any obvious and necessary modifications may be made.

As stated above, withreference to the treatment of the sapwood with the polyhydroxy-benzene solution, for effecting complete penetration of greater thicknesses of the sapwood material with ammonia, the time of exposure of the wood in the atmosphere containing the ammonia may be extended. On the other hand, such complete penetration can also be effected in a shorter time, for example in one or two hours, by employing super-atmospheric pressures.

After allowing the ammonia vapors to penetrate and dissolve in the adsorbed polyhydroxybenzene solution in the wood, the wood may be dried b means of any of the well-known procedures employed in the art, for example, kiln drying or air drying. It is to be noted that the main part of the coloration of the walnut sapwood develops during the treatment with the Volatile alkaline reacting substances such as ammonia. Such coloration may deepen somewhat during the subsequent drying period.

The polyhydroxy-benzene solutions can be used repeatedly with addition of fresh make-up solution to maintain volume and concentration. The reused solutions can be purified by precipitating the dark material present by adjusting the pH to between 2 and 3, and. separating the insoluble matter by decantation or filtration. For example, a repeatedly used pyrogallol solu tion is mixed with about 3 grams'of sulfuric acid per gallon of the solution to bring the pH to about 2.5, and the solution is allowed. to stand for several hours. A dark precipitate settles out from which most of the solution can be decanted in nearly colorless form and without substantial loss of pyrogallol.

The following examples are given by way of illustration and are not intended to constitute a limitation of the invention:

Example I Walnut sapwood in the form of rough green lumber was immersed in a water solution containing 0.3% pyrogallol at a pH of about 4 and a temperature of 75 C., maintained through indirect heating. The wood was kept in this so1ution for two hours.

After such steeping, the wood was removed to a chamber in which air of low ammonia content was maintained. An atmosphere analyzing about 100 mg. of NH: in one liter of air was found to be satisfactory for developing the coloration within a period of one to four hours.

The wood was then moved into a drying chamber and there dried to specified moisture content. After drying it showed a color similar to that of the heartwood to a depth of approximately one-fourth of an inch.

Example II Walnut sapwood, or heartwood containing varying amounts of sapwood, was immersed. in a treating tank containing a water solution of about 0.4% hydroquinone at a pH of 4 to 6. The temperature was maintained at about 70 C., and the time of treating was about two hours. In order to increase stability of the hydroquinone solution, sodium acid sulflte in small amount may be added. For example, addition of 510% sodium sulfite based on the hydroquinone or 02-04% of the total solution was found to be advantageous.

After draining the solution from the treating tank to storage for re-use, the bottom of the tank was covered by a commercial concentrated ammonia solution; the wood was exposed to the vapors developing from this solution for a period of about four hours and then dried in the usual way. A color very similar to that of walnut heartwood resulted. Where heartwood containing sapwood was so treated, the heartwood color was substantially completely unaltered by the treatment.

Example III Green walnut lumber containing sapwood was treated with a solution containing 2% hydroquinone and having a pH of about 5 by brushing this solution on to the surfaces. The lumber was then stacked to allow penetration of the solution into the wood. After one or two days, depending on temperature conditions, the treated wood was exposed to air containing ammonia and subsequently dried. The resulting darkened sapwood closely resembled the shade of heartwood.

Example IV A debarked log of walnut, having an outer layer of sapwood, on an average two inches thick, was immersed in hot water to soften the wood for veneer cutting. Hydroquinone was dissolved in the water in an amount to produce a concentration of about 0.5%. The pH of this solution was maintained at about 4. After keeping the log in this solution for about sixty hours at 30-90" C., it was moved into a chamber containing ammonia vapors and allowed to remain there for live hours. Subsequent cutting indicated that the resulting coloration had completely penetrated the sapwood layer. This coloration was also found to be uniform in shade throughout the original thickness of the log. Thus, the veneer fiitches produced by slicing were of uniform shade resembling that of walnut heartwood.

It was found advantageous in following the procedure of the above example, to keep the air in the gas-treating chamber at high moisture content and temperature, for example, about 180 F. and 95% relative humidity, to avoid cooling and drying the log below the conditions most favorable for veneer cutting.

Example V Results substantially identical with those obtained by the procedure of Example IV above were obtained by substituting for the hydroquinone a mixture of 50% pyrogallol and 50% hydroquinone.

It is a primary advantage of the method of the present invention that it is not limited to treatment of material consisting solely of walnut sapwood. The method of the present invention will not produce any darkening of the shade of the walnut heartwood. Therefore, the method is applicable to material in the form of unfinished lumber or other form which is composed of layers containing both sapwood and heartwood, as set forth in Example II above.

By this precise two step method, the walnut sapwood is produced in a highly desirable color, the shade of which is nearly identical with or very closely similar to the color of walnut heartwood, and such color can be produced to any desired depth. The word ammonia as used in the claim is intended to include ammonia in gaseous form, or in solution in which it exists as ammonium hydroxide.

Having thus described my invention, I claim:

A method of imparting in depth to walnut sapwood a color resembling that of walnut heartwood which consists essentially of immersing for a period of from about 2 to about 60 hours walnut sapwood in an aqueous solution containing from about 0.2% to about 3% by weight of hydroquinone and having a pH of from about 2 to about 5, said solution being maintained at a temperature of from about 70 C. to about C. and thereafter exposing the hydroquinone impregnated sapwood so produced to ammonia vapor.

EDUARD FARBER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PA'I'EN'IS.

OTHER REFERENCES Organic Chemistry by V. von Richter, translation from 11th German edition by DAlbe, published in 1922, by P. Blakistons Son 8; Co., Phila... vol. 2, page 218. 

